Image sensor with improved light sensitivity

ABSTRACT

A system for capturing a color image, includes a two-dimensional array having first and second groups of pixels, pixels from the first group of pixels have narrower spectral photoresponses than pixels from the second group of pixels and the first group of pixels has individual pixels that have spectral photoresponses that correspond to a set of at least two colors; the placement of the first and second groups of pixels defining a pattern that has a minimal repeating unit including at least twelve pixels, and a plurality of non-overlapping cells wherein each cell has at least two pixels representing a specific color selected from the first group of pixels and a plurality of pixels selected from the second group of pixels; a structure for combining pixels of like color from at least two of the plurality of cells within the minimal repeating unit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 11/419,574 filed May22, 2006 now U.S. Pat. No. 7,916,362, of Takayuki Kijima et al, entitled“IMAGE SENSOR WITH IMPROVED LIGHT SENSITIVITY”, the disclosure of whichis incorporated herein.

The present application is related to U.S. Ser. No. 11/191,538 (U.S.Patent Application Publication 2007/0024879), filed Jul. 28, 2005, ofJohn F. Hamilton Jr. and John T. Compton, entitled “PROCESSING COLOR ANDPANCHROMATIC PIXELS”; and

U.S. Ser. No. 11/191,729 (U.S. Patent Application Publication2007/0024931), filed Jul. 28, 2005, of John T. Compton and John F.Hamilton, Jr., entitled “IMAGE SENSOR WITH IMPROVED LIGHT SENSITIVITY”.

FIELD OF THE INVENTION

This invention relates to a two-dimensional image sensor with improvedlight sensitivity.

BACKGROUND OF THE INVENTION

An electronic imaging system depends on an electronic image sensor tocreate an electronic representation of a visual image. Examples of suchelectronic image sensors include charge coupled device (CCD) imagesensors and active pixel sensor (APS) devices (APS devices are oftenreferred to as CMOS sensors because of the ability to fabricate them ina Complementary Metal Oxide Semiconductor process). Typically, theseimages sensors include a number of light sensitive pixels, oftenarranged in a regular pattern of rows and columns. For capturing colorimages, a pattern of filters is typically fabricated on the pattern ofpixels, with different filter materials being used to make individualpixels sensitive to only a portion of the visible light spectrum. Thecolor filters necessarily reduce the amount of light reaching eachpixel, and thereby reduce the light sensitivity of each pixel. A needpersists for improving the light sensitivity, or photographic speed, ofelectronic color image sensors to permit images to be captured at lowerlight levels or to allow images at higher light levels to be capturedwith shorter exposure times.

Image sensors are either linear or two-dimensional. Generally, thesesensors have two different types of applications. The two-dimensionalsensors are typically suitable for image capture devices such as digitalcameras, cell phones and other applications. Linear sensors are oftenused for scanning documents. In either case, when color filters areemployed the image sensors have reduced sensitivity.

A linear image sensor, the KLI-4104 manufactured by Eastman KodakCompany, includes four linear, single pixel wide arrays of pixels, withcolor filters applied to three of the arrays to make each arraysensitive to either red, green, or blue in its entirety, and with nocolor filter array applied to the fourth array; furthermore, the threecolor arrays have larger pixels to compensate for the reduction in lightsensitivity due to the color filters, and the fourth array has smallerpixels to capture a high resolution monochrome image. When an image iscaptured using this image sensor, the image is represented as a highresolution, high photographic sensitivity monochrome image along withthree lower resolution images with roughly the same photographicsensitivity and with each of the three images corresponding to eitherred, green, or blue light from the image; hence, each point in theelectronic image includes a monochrome value, a red value, a greenvalue, and a blue value. However, since this is a linear image sensor,it requires relative mechanical motion between the image sensor and theimage in order to scan the image across the four linear arrays ofpixels. This limits the speed with which the image is scanned andprecludes the use of this sensor in a handheld camera or in capturing ascene that includes moving objects.

There is also known in the art an electronic imaging system described inU.S. Pat. No. 4,823,186 by Akira Muramatsu that includes two sensors,wherein each of the sensors includes a two-dimensional array of pixelsbut one sensor has no color filters and the other sensor includes apattern of color filters included with the pixels, and with an opticalbeam splitter to provide each image sensor with the image. Since thecolor sensor has a pattern of color filters applied, each pixel in thecolor sensor provides only a single color. When an image is capturedwith this system, each point in the electronic image includes amonochrome value and one color value, and the color image must have themissing colors at each pixel location interpolated from the nearbycolors. Although this system improves the light sensitivity over asingle conventional image sensor, the overall complexity, size, and costof the system is greater due to the need for two sensors and a beamsplitter. Furthermore, the beam splitter directs only half the lightfrom the image to each sensor, limiting the improvement in photographicspeed.

In addition to the linear image sensor mentioned above, there are knownin the art image sensors with two-dimensional arrays of pixels where thepixels include pixels that do not have color filters applied to them.For example, see Sato, et al. in U.S. Pat. No. 4,390,895, Yamagami, etal. in U.S. Pat. No. 5,323,233, and Gindele, et al. in U.S. Pat. No.6,476,865. In each of the cited patents, the sensitivity of theunfiltered or monochrome pixels is significantly higher than the colorpixels, requiring the application of gain to the color pixels in orderto match the color and monochrome signals from the pixel array.Increasing gain increases noise as well as signal, causing degradationin the overall signal to noise ratio of the resulting image. Frame in USPatent Application 2003/0210332 discloses a pixel array with most of thepixels being unfiltered, but the color pixels suffer from the samesensitivity deficit as mentioned above.

Therefore, there persists a need for improving the light sensitivity forelectronic capture devices that employ a single sensor with atwo-dimensional array of pixels.

SUMMARY OF THE INVENTION

The present invention is directed to providing an image sensor having atwo-dimensional array of color and panchromatic pixels that provideshigh sensitivity and is effective in producing full color images.

Briefly summarized, according to one aspect of the present invention,the invention provides a system for capturing a color image, comprising:

a) a two-dimensional array having first and second groups of pixelswherein pixels from the first group of pixels have narrower spectralphotoresponses than pixels from the second group of pixels and whereinthe first group of pixels has individual pixels that have spectralphotoresponses that correspond to a set of at least two colors;

b) the placement of the first and second groups of pixels defining apattern that has a minimal repeating unit including at least twelvepixels, the minimal repeating unit having a plurality of non-overlappingcells wherein each cell has at least two pixels representing a specificcolor selected from the first group of pixels and a plurality of pixelsselected from the second group of pixels arranged to permit thereproduction of a captured color image under different lightingconditions; and

c) means for combining pixels of like color from at least two of theplurality of cells within the minimal repeating unit.

Image sensors in accordance with the present invention are particularlysuitable for low level lighting conditions, where such low levellighting conditions are the result of low scene lighting, short exposuretime, small aperture, or other restriction on light reaching the sensor.They have a broad application and numerous types of image capturedevices can effectively use these sensors.

These and other aspects, objects, features and advantages of the presentinvention will be more clearly understood and appreciated from a reviewof the following detailed description of the preferred embodiments andappended claims, and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional digital still camera systemthat can employ a conventional sensor and processing methods or thesensor and processing methods of the current invention;

FIG. 2 (prior art) is a conventional Bayer color filter array patternshowing a minimal repeating unit and a non-minimal repeating unit;

FIG. 3 provides representative spectral quantum efficiency curves forred, green, and blue pixels, as well as a wider spectrum panchromaticquantum efficiency, all multiplied by the transmission characteristicsof an infrared cut filter;

FIGS. 4A-D provides minimal repeating units for several variations of acolor filter array pattern of the present invention that has colorpixels with the same color photo response arranged in rows or columns;

FIG. 5 shows the cell structure of the minimal repeating unit from FIG.4A;

FIG. 6A is the interpolated panchromatic image for FIG. 4A;

FIG. 6B is the low-resolution color image corresponding to the cells inFIG. 4A and FIG. 5;

FIGS. 7A-C shows several ways of combining the pixels of FIG. 4A;

FIGS. 8A-E provides a minimal repeating unit of six pixels for a colorfilter array pattern of the present invention including several tilingarrangements and an alternative orientation for the minimal repeatingunit;

FIG. 9A-C provides several minimal repeating units for color filterarray patterns of the present invention that are variants of the minimalrepeating unit of FIG. 8;

FIGS. 10A-F provides a minimal repeating unit of eight pixels for acolor filter array pattern of the present invention and includes atiling arrangement and variations with color pixels that havealternative color photoresponse characteristics, including primarycolor, complementary color, three color, and four color alternatives;

FIGS. 11A-B provides a minimal repeating unit for a color filter arrayof the present invention in which more than half the pixels have apanchromatic photoresponse;

FIGS. 12A-B provides a minimal repeating unit and for a color filterarray of the present invention in which the pixels are on a rectangulargrid that is rotated forty-five degrees, and includes a tilingarrangement;

FIGS. 13A-B provides a minimal repeating unit and for a color filterarray of the present invention in which the pixels are arranged in ahexagonal pattern, and includes a tiling arrangement;

FIG. 14 provides a minimal repeating unit for a color filter array ofthe present invention that is an alternative to FIG. 13;

FIG. 15 provides a minimal repeating unit for a color filter array ofthe present invention that is an alternative to FIG. 13;

FIG. 16 is the minimal repeating unit of FIG. 4A with subscripts forindividual pixels within the minimal repeating unit;

FIGS. 17A-E shows the panchromatic pixels and the color pixels of onecell of FIG. 16, and various ways in which the color pixels arecombined;

FIG. 18 is a process diagram of the present invention showing the methodof processing the color and panchromatic pixel data from a sensor of thepresent invention;

FIGS. 19A-D illustrates methods of the present invention forinterpolating missing colors in the low-resolution partial color imageof FIG. 18;

FIG. 20 provides two minimal repeating units of the type shown in FIG.8A showing combining pixels between adjacent minimal repeating units aswell as within of the minimal repeating units;

FIGS. 21A-D each provide two minimal repeating units that are a variantof FIG. 10A showing several ways of combining pixels betweenhorizontally adjacent minimal repeating units and within minimalrepeating units;

FIG. 22 provides two minimal repeating units that are a variant of FIG.10A showing combining pixels between vertically adjacent minimalrepeating units;

FIG. 23 provides three minimal repeating units that are a variant ofFIG. 10A showing combining pixels from three horizontally adjacentminimal repeating units;

FIG. 24 provides five minimal repeating units that are a variant of FIG.10A showing combining pixels from five horizontally adjacent minimalrepeating units;

FIGS. 25A-B provides several minimal repeating units that are a variantof FIG. 10A showing multiple overlapping groups of minimal repeatingunits for the purpose of combining pixels from adjacent minimalrepeating units;

FIGS. 26A-B provides several minimal repeating units of the type shownin FIG. 8A showing combining pixels from multiple mutually adjacentminimal repeating units;

FIG. 27 provides a minimal repeating unit that is a variant of FIG. 10Ashowing combining color pixels with panchromatic pixels;

FIG. 28 provides two minimal repeating units that are a variation of thepresent invention that include cells showing combining pixels betweenminimal repeating units, between cells, and within cells, as well ascombining color and panchromatic pixels; and

FIGS. 29A-C provides a minimal repeating unit that is a variation of thepresent invention that includes cells showing specific examples ofcombining pixels between minimal repeating units, between cells, andwithin cells.

DETAILED DESCRIPTION OF THE INVENTION

Because digital cameras employing imaging devices and related circuitryfor signal capture and correction and for exposure control are wellknown, the present description will be directed in particular toelements forming part of, or cooperating more directly with, method andapparatus in accordance with the present invention. Elements notspecifically shown or described herein are selected from those known inthe art. Certain aspects of the embodiments to be described are providedin software. Given the system as shown and described according to theinvention in the following materials, software not specifically shown,described or suggested herein that is useful for implementation of theinvention is conventional and within the ordinary skill in such arts.

Turning now to FIG. 1, a block diagram of an image capture device shownas a digital camera embodying the present invention is shown. Although adigital camera will now be explained, the present invention is clearlyapplicable to other types of image capture devices. In the disclosedcamera, light 10 from the subject scene is input to an imaging stage 11,where the light is focused by lens 12 to form an image on solid stateimage sensor 20. Image sensor 20 converts the incident light to anelectrical signal for each picture element (pixel). The image sensor 20of the preferred embodiment is a charge coupled device (CCD) type or anactive pixel sensor (APS) type (APS devices are often referred to asCMOS sensors because of the ability to fabricate them in a ComplementaryMetal Oxide Semiconductor process). Other types of image sensors havingtwo-dimensional array of pixels are used provided that they employ thepresent invention. The present invention also makes use of an imagesensor 20 having a two-dimensional array of color and panchromaticpixels as will become clear later in this specification after FIG. 1 isdescribed. Examples of the patterns of color and panchromatic pixels ofthe present invention that are used with the image sensor 20 are seen inFIGS. 4A-D, FIG. 8A, FIG. 8E, FIGS. 9A-C, FIG. 10A, FIGS. 10C-F, FIGS.11A-B, FIG. 12, FIG. 13, FIG. 14, and FIG. 15, although other patternsare used within the spirit of the present invention.

The amount of light reaching the sensor 20 is regulated by an iris block14 that varies the aperture and the neutral density (ND) filter block 13that includes one or more ND filters interposed in the optical path.Also regulating the overall light level is the time that the shutterblock 18 is open. The exposure controller block 40 responds to theamount of light available in the scene as metered by the brightnesssensor block 16 and controls all three of these regulating functions.

This description of a particular camera configuration will be familiarto one skilled in the art, and it will be obvious that many variationsand additional features are present. For example, an autofocus system isadded, or the lenses are detachable and interchangeable. It will beunderstood that the present invention is applied to any type of digitalcamera, where similar functionality is provided by alternativecomponents. For example, the digital camera is a relatively simple pointand shoot digital camera, where the shutter 18 is a relatively simplemovable blade shutter, or the like, instead of the more complicatedfocal plane arrangement. The present invention can also be practiced onimaging components included in non-camera devices such as mobile phonesand automotive vehicles.

The analog signal from image sensor 20 is processed by analog signalprocessor 22 and applied to analog to digital (A/D) converter 24. Timinggenerator 26 produces various clocking signals to select rows and pixelsand synchronizes the operation of analog signal processor 22 and A/Dconverter 24. The image sensor stage 28 includes the image sensor 20,the analog signal processor 22, the A/D converter 24, and the timinggenerator 26. The components of image sensor stage 28 are separatelyfabricated integrated circuits, or they are fabricated as a singleintegrated circuit as is commonly done with CMOS image sensors. Theresulting stream of digital pixel values from A/D converter 24 is storedin memory 32 associated with digital signal processor (DSP) 36.

Digital signal processor 36 is one of three processors or controllers inthis embodiment, in addition to system controller 50 and exposurecontroller 40. Although this partitioning of camera functional controlamong multiple controllers and processors is typical, these controllersor processors are combined in various ways without affecting thefunctional operation of the camera and the application of the presentinvention. These controllers or processors can comprise one or moredigital signal processor devices, microcontrollers, programmable logicdevices, or other digital logic circuits. Although a combination of suchcontrollers or processors has been described, it should be apparent thatan alternative embodiment designates one controller or processor toperform all of the needed functions. All of these variations can performthe same function and fall within the scope of this invention, and theterm “processing stage” will be used as needed to encompass all of thisfunctionality within one phrase, for example, as in processing stage 38in FIG. 1.

In the illustrated embodiment, DSP 36 manipulates the digital image datain its memory 32 according to a software program permanently stored inprogram memory 54 and copied to memory 32 for execution during imagecapture. DSP 36 executes the software necessary for practicing imageprocessing shown in FIG. 18. Memory 32 includes any type of memory, suchas SDRAM. A bus 30 comprising a pathway for address and data signalsconnects DSP 36 to its related memory 32, A/D converter 24 and otherrelated devices.

System controller 50 controls the overall operation of the camera basedon a software program stored in program memory 54, which can includeFlash EEPROM or other nonvolatile memory. This memory is also used tostore image sensor calibration data, user setting selections and otherdata which must be preserved when the camera is turned off. Systemcontroller 50 controls the sequence of image capture by directingexposure controller 40 to operate the lens 12, ND filter 13, iris 14,and shutter 18 as previously described, directing the timing generator26 to operate the image sensor 20 and associated elements, and directingDSP 36 to process the captured image data. After an image is capturedand processed, the final image file stored in memory 32 is transferredto a host computer via interface 57, stored on a removable memory card64 or other storage device, and displayed for the user on image display88.

A bus 52 includes a pathway for address, data and control signals, andconnects system controller 50 to DSP 36, program memory 54, systemmemory 56, host interface 57, memory card interface 60 and other relateddevices. Host interface 57 provides a high speed connection to apersonal computer (PC) or other host computer for transfer of image datafor display, storage, manipulation or printing. This interface is anIEEE1394 or USB2.0 interface or any other suitable digital interface.Memory card 64 is typically a Compact Flash (CF) card inserted intosocket 62 and connected to the system controller 50 via memory cardinterface 60. Other types of storage that are utilized include withoutlimitation PC-Cards, MultiMedia Cards (MMC), or Secure Digital (SD)cards.

Processed images are copied to a display buffer in system memory 56 andcontinuously read out via video encoder 80 to produce a video signal.This signal is output directly from the camera for display on anexternal monitor, or processed by display controller 82 and presented onimage display 88. This display is typically an active matrix colorliquid crystal display (LCD), although other types of displays are usedas well.

A user control and interface status 68, including all or any combinationof viewfinder display 70, exposure display 72, status display 76, imagedisplay 88, and user inputs 74, is controlled by a combination ofsoftware programs executed on exposure controller 40 and systemcontroller 50. User inputs 74 typically include some combination ofbuttons, rocker switches, joysticks, rotary dials or touchscreens.Exposure controller 40 operates light metering, exposure mode, autofocusand other exposure functions. The system controller 50 manages thegraphical user interface (GUI) presented on one or more of the displays,e.g., on image display 88. The GUI typically includes menus for makingvarious option selections and review modes for examining capturedimages.

Exposure controller 40 accepts user inputs selecting exposure mode, lensaperture, exposure time (shutter speed), and exposure index or ISO speedrating and directs the lens and shutter accordingly for subsequentcaptures. Brightness sensor 16 is employed to measure the brightness ofthe scene and provide an exposure meter function for the user to referto when manually setting the ISO speed rating, aperture and shutterspeed. In this case, as the user changes one or more settings, the lightmeter indicator presented on viewfinder display 70 tells the user towhat degree the image will be over or underexposed. In an automaticexposure mode, the user changes one setting and the exposure controller40 automatically alters another setting to maintain correct exposure,e.g., for a given ISO speed rating when the user reduces the lensaperture the exposure controller 40 automatically increases the exposuretime to maintain the same overall exposure.

The ISO speed rating is an important attribute of a digital stillcamera. The exposure time, the lens aperture, the lens transmittance,the level and spectral distribution of the scene illumination, and thescene reflectance determine the exposure level of a digital stillcamera. When an image from a digital still camera is obtained using aninsufficient exposure, proper tone reproduction can generally bemaintained by increasing the electronic or digital gain, but the imagewill contain an unacceptable amount of noise. As the exposure isincreased, the gain is decreased, and therefore the image noise cannormally be reduced to an acceptable level. If the exposure is increasedexcessively, the resulting signal in bright areas of the image canexceed the maximum signal level capacity of the image sensor or camerasignal processing. This can cause image highlights to be clipped to forma uniformly bright area, or to bloom into surrounding areas of theimage. It is important to guide the user in setting proper exposures. AnISO speed rating is intended to serve as such a guide. In order to beeasily understood by photographers, the ISO speed rating for a digitalstill camera should directly relate to the ISO speed rating forphotographic film cameras. For example, if a digital still camera has anISO speed rating of ISO 200, then the same exposure time and apertureshould be appropriate for an ISO 200 rated film/process system.

The ISO speed ratings are intended to harmonize with film ISO speedratings. However, there are differences between electronic andfilm-based imaging systems that preclude exact equivalency. Digitalstill cameras can include variable gain, and can provide digitalprocessing after the image data has been captured, enabling tonereproduction to be achieved over a range of camera exposures. It istherefore possible for digital still cameras to have a range of speedratings. This range is defined as the ISO speed latitude. To preventconfusion, a single value is designated as the inherent ISO speedrating, with the ISO speed latitude upper and lower limits indicatingthe speed range, that is, a range including effective speed ratings thatdiffer from the inherent ISO speed rating. With this in mind, theinherent ISO speed is a numerical value calculated from the exposureprovided at the focal plane of a digital still camera to producespecified camera output signal characteristics. The inherent speed isusually the exposure index value that produces peak image quality for agiven camera system for normal scenes, where the exposure index is anumerical value that is inversely proportional to the exposure providedto the image sensor.

The foregoing description of a digital camera will be familiar to oneskilled in the art. It will be obvious that there are many variations ofthis embodiment that are possible and are selected to reduce the cost,add features or improve the performance of the camera. The followingdescription will disclose in detail the operation of this camera forcapturing images according to the present invention. Although thisdescription is with reference to a digital camera, it will be understoodthat the present invention applies for use with any type of imagecapture device having an image sensor with color and panchromaticpixels.

The image sensor 20 shown in FIG. 1 typically includes a two-dimensionalarray of light sensitive pixels fabricated on a silicon substrate thatprovide a way of converting incoming light at each pixel into anelectrical signal that is measured. As the sensor is exposed to light,free electrons are generated and captured within the electronicstructure at each pixel. Capturing these free electrons for some periodof time and then measuring the number of electrons captured, ormeasuring the rate at which free electrons are generated, allows thelight level at each pixel to be measured. In the former case,accumulated charge is shifted out of the array of pixels to a charge tovoltage measurement circuit as in a charge coupled device (CCD), or thearea close to each pixel contains elements of a charge to voltagemeasurement circuit as in an active pixel sensor (APS or CMOS sensor).

Whenever general reference is made to an image sensor in the followingdescription, it is understood to be representative of the image sensor20 from FIG. 1. It is further understood that all examples and theirequivalents of image sensor architectures and pixel patterns of thepresent invention disclosed in this specification are used for imagesensor 20.

In the context of an image sensor, a pixel (a contraction of “pictureelement”) refers to a discrete light sensing area and charge shifting orcharge measurement circuitry associated with the light sensing area. Inthe context of a digital color image, the term pixel commonly refers toa particular location in the image having associated color values.

In order to produce a color image, the array of pixels in an imagesensor typically has a pattern of color filters placed over them. FIG. 2shows a pattern of red, green, and blue color filters that is commonlyused. This particular pattern is commonly known as a Bayer color filterarray (CFA) after its inventor Bryce Bayer as disclosed in U.S. Pat. No.3,971,065. This pattern is effectively used in image sensors having atwo-dimensional array of color pixels. As a result, each pixel has aparticular color photoresponse that, in this case, is a predominantsensitivity to red, green or blue light. Another useful variety of colorphotoresponses is a predominant sensitivity to magenta, yellow, or cyanlight. In each case, the particular color photoresponse has highsensitivity to certain portions of the visible spectrum, whilesimultaneously having low sensitivity to other portions of the visiblespectrum. The term color pixel will refer to a pixel having a colorphotoresponse.

The set of color photoresponses selected for use in a sensor usually hasthree colors, as shown in the Bayer CFA, but it can also include four ormore. As used herein, a panchromatic photoresponse refers to aphotoresponse having a wider spectral sensitivity than those spectralsensitivities represented in the selected set of color photoresponses. Apanchromatic photosensitivity can have high sensitivity across theentire visible spectrum. The term panchromatic pixel will refer to apixel having a panchromatic photoresponse. Although the panchromaticpixels generally have a wider spectral sensitivity than the set of colorphotoresponses, each panchromatic pixel can have an associated filter.Such filter is either a neutral density filter or a color filter.

When a pattern of color and panchromatic pixels is on the face of animage sensor, each such pattern has a repeating unit that is acontiguous subarray of pixels that acts as a basic building block. Byjuxtaposing multiple copies of the repeating unit, the entire sensorpattern is produced. The juxtaposition of the multiple copies ofrepeating units is done in diagonal directions as well as in thehorizontal and vertical directions.

A minimal repeating unit is a repeating unit such that no otherrepeating unit has fewer pixels. For example, the CFA in FIG. 2 includesa minimal repeating unit that is two pixels by two pixels as shown bypixel block 100 in FIG. 2. Multiple copies of this minimal repeatingunit are tiled to cover the entire array of pixels in an image sensor.The minimal repeating unit is shown with a green pixel in the upperright corner, but three alternative minimal repeating units can easilybe discerned by moving the heavy outlined area one pixel to the right,one pixel down, or one pixel diagonally to the right and down. Althoughpixel block 102 is a repeating unit, it is not a minimal repeating unitbecause pixel block 100 is a repeating unit and block 100 has fewerpixels than block 102.

An image captured using an image sensor having a two-dimensional arraywith the CFA of FIG. 2 has only one color value at each pixel. In orderto produce a full color image, there are a number of techniques forinferring or interpolating the missing colors at each pixel. These CFAinterpolation techniques are well known in the art and reference is madeto the following patents: U.S. Pat. No. 5,506,619, U.S. Pat. No.5,629,734, and U.S. Pat. No. 5,652,621.

FIG. 3 shows the relative spectral sensitivities of the pixels with red,green, and blue color filters in a typical camera application. TheX-axis in FIG. 3 represents light wavelength in nanometers, and theY-axis represents efficiency. In FIG. 3, curve 110 represents thespectral transmission characteristic of a typical filter used to blockinfrared and ultraviolet light from reaching the image sensor. Such afilter is needed because the color filters used for image sensorstypically do not block infrared light, hence the pixels are unable todistinguish between infrared light and light that is within thepassbands of their associated color filters. The infrared blockingcharacteristic shown by curve 110 prevents infrared light fromcorrupting the visible light signal. The spectral quantum efficiency,i.e. the proportion of incident photons that are captured and convertedinto a measurable electrical signal, for a typical silicon sensor withred, green, and blue filters applied is multiplied by the spectraltransmission characteristic of the infrared blocking filter representedby curve 110 to produce the combined system quantum efficienciesrepresented by curve 114 for red, curve 116 for green, and curve 118 forblue. It is understood from these curves that each color photoresponseis sensitive to only a portion of the visible spectrum. By contrast, thephotoresponse of the same silicon sensor that does not have colorfilters applied (but including the infrared blocking filtercharacteristic) is shown by curve 112; this is an example of apanchromatic photoresponse. By comparing the color photoresponse curves114, 116, and 118 to the panchromatic photoresponse curve 112, it isclear that the panchromatic photoresponse is three to four times moresensitive to wide spectrum light than any of the color photoresponses.

The greater panchromatic sensitivity shown in FIG. 3 permits improvingthe overall sensitivity of an image sensor by intermixing pixels thatinclude color filters with pixels that do not include color filters.However, the color filter pixels will be significantly less sensitivethan the panchromatic pixels. In this situation, if the panchromaticpixels are properly exposed to light such that the range of lightintensities from a scene cover the full measurement range of thepanchromatic pixels, then the color pixels will be significantlyunderexposed. Hence, it is advantageous to adjust the sensitivity of thecolor filter pixels so that they have roughly the same sensitivity asthe panchromatic pixels. The sensitivity of the color pixels areincreased, for example, by increasing the size of the color pixelsrelative to the panchromatic pixels, with an associated reduction inspatial pixels.

FIG. 4A represents a two-dimensional array of pixels having two groups.Pixels from the first group of pixels have a narrower spectralphotoresponse than pixels from the second group of pixels. The firstgroup of pixels includes individual pixels that relate to at least twodifferent spectral photoresponses corresponding to at least two colorfilters. These two groups of pixels are intermixed to improve theoverall sensitivity of the sensor. As will become clearer in thisspecification, the placement of the first and second groups of pixelsdefines a pattern that has a minimal repeating unit including at leasttwelve pixels. The minimal repeating unit includes first and secondgroups of pixels arranged to permit the reproduction of a captured colorimage under different lighting conditions.

The complete pattern shown in FIG. 4A represents a minimal repeatingunit that is tiled to cover an entire array of pixels. As with FIG. 2,there are several other minimal repeating units that are used todescribe this overall arrangement of color and panchromatic pixels, butthey are all essentially equivalent in their characteristics and each isa subarray of pixels, the subarray being eight pixels by eight pixels inextent. An important feature of this pattern is alternating rows ofpanchromatic and color pixels with the color rows having pixels with thesame color photoresponse grouped together. The groups of pixels with thesame photoresponse along with some of their neighboring panchromaticpixels are considered to form four cells that make up the minimalrepeating unit, a cell being a contiguous subarray of pixels havingfewer pixels than a minimal repeating unit.

These four cells, delineated by heavy lines in FIG. 4A and shown ascells 120, 122, 124, and 126 in FIG. 5, enclose four groups offour-by-four pixels each, with 120 representing the upper left cell, 122representing the upper right cell, 124 representing the lower left cell,and 126 representing the lower right cell. Each of the four cellsincludes eight panchromatic pixels and eight color pixels of the samecolor photoresponse. The color pixels in a cell are combined torepresent the color for that entire cell. Hence, cell 120 in FIG. 5 isconsidered to be a green cell, cell 122 is considered to be a red cell,and so on. Each cell includes at least two pixels of the same color,thereby allowing pixels of the same color to be combined to overcome thedifference in photosensitivity between the color pixels and thepanchromatic pixels.

In the case of a minimal repeating unit with four non-overlapping cells,with each cell having two pixels of the same color and two panchromaticpixels, it is clear that the minimal repeating unit includes sixteenpixels. In the case of a minimal repeating unit with threenon-overlapping cells, with each cell having two pixels of the samecolor and two panchromatic pixels, it is clear that the minimalrepeating unit includes twelve pixels.

In accordance with the present invention, the minimal repeating unit ofFIG. 4A, when considered in light of the cell structure identified inFIG. 5, can represent the combination of a high-resolution panchromaticimage and a low-resolution Bayer pattern color image arranged to permitthe reproduction of a captured color image under different lightingconditions. The individual elements of the Bayer pattern image representthe combination of the color pixels in the corresponding cells. Thefirst group of pixels defines a low-resolution color filter array imageand the second group of pixels defines a high-resolution panchromaticimage. See FIG. 6A and FIG. 6B. FIG. 6A represents the high-resolutionpanchromatic image corresponding to FIG. 4A, including both thepanchromatic pixels P from FIG. 4A as well as interpolated panchromaticpixels P′; and FIG. 6B represents the low-resolution Bayer pattern colorimage, with R′, G′, and B′ representing for each of the cells outlinedin FIG. 5 the cell color associated with the combined color pixels inthe cell.

In the following discussion, all cells in FIGS. 4B-D are delineated byheavy lines, as they were in FIG. 4A.

In addition to alternative minimal repeating units of FIG. 4A, each cellof the pattern is rotated 90 degrees to produce the pattern shown inFIG. 4B. This is substantially the same pattern, but it places thehighest panchromatic sampling frequency in the vertical directioninstead of the horizontal direction. The choice to use FIG. 4A or FIG.4B depends on whether or not it is desired to have higher panchromaticspatial sampling in either the horizontal or vertical directionsrespectively. However, it is clear that the resulting cells that make upthe minimal repeating unit in both patterns produce the samelow-resolution color image for both patterns. Hence, FIG. 4A and FIG. 4Bare equivalent from a color perspective. In general, FIG. 4A and FIG. 4Bare examples of practicing the present invention with the panchromaticpixels arranged linearly in either rows or columns. Furthermore, FIG. 4Ahas single rows of panchromatic pixels with each row separated from aneighboring row of panchromatic pixels by a row of color pixels; FIG. 4Bhas the same characteristic in the column direction.

FIG. 4C represents yet another alternative minimal repeating unit toFIG. 4A with essentially the same cell color characteristics. However,FIG. 4C shows the panchromatic and color rows staggered on a cell bycell basis. This can improve the vertical panchromatic resolution. Yetanother alternative minimal repeating unit to FIG. 4A is represented inFIG. 4D, wherein the panchromatic and color rows are staggered by columnpairs. This also has the potential of improving the verticalpanchromatic resolution. A characteristic of all of the minimalrepeating units of FIGS. 4A-D is that groups of two or more same colorpixels are arranged side by side in either rows or columns.

FIGS. 4A-D all have the same color structure with the cells thatconstitute the minimal repeating unit expressing a low-resolution Bayerpattern. It can therefore be seen that a variety of arrangements ofpanchromatic pixels and grouped color pixels are constructed within thespirit of the present invention.

In order to increase the color photosensitivity to overcome thedisparity between the panchromatic photosensitivity and the colorphotosensitivity, the color pixels within each cell are combined invarious ways. For example, the charge from same colored pixels iscombined or binned in a CCD image sensor or in types of active pixelsensors that permit binning (see FIG. 1, image sensor 20).Alternatively, the voltages corresponding to the measured amounts ofcharge in same colored pixels are averaged, for example by connecting inparallel capacitors that are charged to these voltages (see FIG. 1,image sensor 20). In the case of averaging voltages by connecting inparallel capacitors, the capacitors can be of equal sizes to do a simpleaverage, or they can be of different sizes in order to do a weightedaverage. In yet another approach, the digital representations of thelight levels at same colored pixels are summed, averaged, or digitallyfiltered to provide a combined result, for example in FIG. 1, digitalsignal processor 36. Combining or binning charge from two pixels doublesthe signal level, while the noise associated with sampling and readingout the combined signal remains the same, thereby increasing the signalto noise ratio by a factor of two, representing a corresponding twotimes increase in the photosensitivity of the combined pixels. In thecase of summing the digital representations of the light levels from twopixels, the resulting signal increases by a factor of two, but thecorresponding noise levels from reading the two pixels combine inquadrature, thereby increasing the noise by the square root of two; theresulting signal to noise ratio of the combined pixels thereforeincreases by the square root of two over the uncombined signals. Asimilar analysis applies to voltage or digital averaging.

The previously mentioned approaches for combining signals from samecolored pixels within a cell are used singly or in combinations. Forexample, vertically combining the charge from same colored pixels inFIG. 4A in groups of two produces the combined pixels with combinedsignals R′, G′, and B′ shown in FIG. 7A. In this case, each R′, G′, andB′ has twice the sensitivity of the uncombined pixels. Alternatively,horizontally combining by summing the measured values (either voltage ordigital) from same colored pixels in FIG. 4A in groups of four producesthe combined pixels with combined signals R′, G′, and B′ shown in FIG.7B. In this case, since the signal increases by a factor of four but thenoise increases by 2, each R′, G′, and B′ has twice the sensitivity ofthe uncombined pixels. In another alternative combination scheme,vertically combining the charge from same colored pixels in groups oftwo as in FIG. 7A, and horizontally summing or averaging the measuredvalues of the combined pixels of FIG. 7A in groups of four produces thefinal combined color pixels of FIG. 7C, with R″, G″, and B″ representingthe final combinations of same colored pixels. In this combinationarrangement, the final combined color pixels of FIG. 7C each have fourtimes the sensitivity of the uncombined pixels. Some sensorarchitectures, notably certain CCD arrangements, can permit the chargefrom all eight same colored pixels within each cell to be combined inthe fashion of FIG. 7C, leading to an eightfold increase in sensitivityfor the combined color pixels.

From the foregoing, it will now be understood that there are severaldegrees of freedom in combining color pixels for the purpose ofadjusting the photosensitivity of the color pixels. Well known combiningschemes will suggest themselves to one skilled in the art that are basedon scene content, scene illuminant, overall light level, or othercriteria. Furthermore, the combining scheme is selected to deliberatelypermit the combined pixels to have either less sensitivity or moresensitivity than the panchromatic pixels. The various ways of combiningpixels discussed above are used with image sensors employing any of thepatterns described in this disclosure and related disclosures.

To this point the image sensor has been described as employing red,green, and blue filters such that there are two green pixels for everyred and blue pixel. The present invention is also practiced with red,green, and blue filters in equal proportions as shown in FIG. 8A. Theminimal repeating unit of FIG. 8A is used to tile the sensor array inseveral different ways, some of which are shown in FIGS. 8B-D. It willbe understood that geometrically similar variations of these patterns,such as the minimal repeating unit of FIG. 8A, can be used. FIG. 8Eshows a rotated form of the minimal repeating unit of FIG. 8A.

The present invention is also usable with pixels having more than threecolor photoresponses. FIG. 9A shows a variation of minimal repeating ofFIG. 8A that uses four colors in addition to the panchromatic pixels.FIGS. 9B-C show additional variations of both of these patterns in whichthe single row of panchromatic pixels is replaced by a double row ofpanchromatic pixels. All of these patterns do not have a plurality ofpixels of the same color. This fact and the preferred method for usingsuch patterns will be discussed later.

Another minimal repeating unit is shown in FIG. 10A that contains onered, two green, and one blue pixels. A tiling example, using thispattern, is shown in FIG. 10B.

Image sensors employing cyan, magenta, and yellow sensors are well knownin the art, and the present invention is practiced with cyan, magenta,and yellow color filters. FIG. 10C shows the cyan, magenta, and yellowequivalent of FIG. 10A, with C representing cyan pixels, M representingmagenta pixels, and Y representing yellow pixels.

FIG. 10D shows a minimal repeating unit of the present invention thatincludes cyan pixels (represented by C), magenta pixels (represented byM), yellow pixels (represented by Y), and green pixels (represented byG). FIG. 10E shows yet another alternative four color arrangementincluding red pixels (represented by R), blue pixels (represented by B),green pixels with one color photoresponse (represented by G), andalternative green pixels with a different color photoresponse(represented by E). FIG. 10F shows yet another alternative four colorarrangement, wherein one of the green cells of FIG. 10A is replaced by ayellow cell, with the yellow pixels represented by Y.

FIG. 11A shows a variation of the pattern of FIG. 10A in which each rowof panchromatic pixels is replaced by a double row of panchromaticpixels. An additional example, shown in FIG. 11B, is the same variationapplied to the pattern of FIG. 10E.

The present invention is practiced with pixels arrays other than arectangular array. FIG. 12A shows a variation of the pattern of FIG. 8Ain which the pixels are octagonal and are arranged on a diagonal row.Because the pixel geometry is octagonal, there are small squarevacancies located between horizontal and vertical neighbors that can beused for required sensor functionality such as data transfer circuitry.FIG. 12B shows an example of a tiling pattern using the minimalrepeating unit of FIG. 12A. In FIG. 12B the panchromatic pixels appearin rows that are diagonal in nature. Likewise, the color pixels alsoappear in diagonally oriented rows.

FIG. 13A shows another variation of the pattern of FIG. 8A in which thepixels are hexagonal and arranged vertically. FIG. 13B shows an exampleof a tiling pattern using the minimal repeating unit of FIG. 13A. InFIG. 13B the panchromatic pixels appear in columns Likewise, the colorpixels also appear in columns.

FIG. 14 shows another minimal repeating unit using fewer panchromaticpixels than color pixels wherein the pixels are hexagonally packed andwherein the panchromatic pixels appear in rows that are diagonal innature. Also, in FIG. 14, the color pixels appear in diagonally orientedrows. FIG. 15 shows another variation of the pattern of FIG. 13A. Withinthe scope of the present invention, it should be noted that rows andcolumns of pixels are not necessarily perpendicular to each other as isshown in FIGS. 12A-15.

Turning now to FIG. 16, the minimal repeating unit of FIG. 5 is shownsubdivided into four cells, a cell being a contiguous subarray of pixelshaving fewer pixels than a minimal repeating unit. The software neededto provide the following processing is included in DSP 36 of FIG. 1.Cells 220, 224, 226, and 228 are examples of cells wherein these cellscontain pixels having green, red, blue and green photoresponses,respectively. In this example, cell 220 contains both panchromaticpixels and green pixels, the green pixels being identified as pixelgroup 222. The eventual goal is to produce a single green signal forcell 220 by combining the eight green signals from the green pixels inpixel group 222. Depending on the image sensor's mode of operation, asingle green signal is produced by combining all eight green signals inthe analog domain (e.g. by charge binning), or multiple green signalsare produced by combining smaller groups of pixels taken from pixelgroup 222. The panchromatic pixels of cell 220 are shown in FIG. 17A. Inthe following examples, all eight signals from these panchromatic pixelsare individually digitized. The green pixels of cell 220 are shown inFIGS. 17B-17E wherein they are grouped together according to how theirsignals are combined. FIG. 17B depicts the case in which all eight greenpixels are combined to produce a single green signal for cell 220 (FIG.16). The sensor can produce two green signals, for example, by firstcombining the signals from pixels G21, G22, G23, and G24, and thencombining the signals from pixels G41, G42, G43, and G44, as shown inFIG. 17C. Two signals are produced in other ways as well. The sensor canfirst combine signals from pixels G21, G22, G41, and G42, and thencombine signals from pixels G23, G24, G43, and G44, as shown in FIG.17D. The sensor can also produce four green signals for cell 220 bycombining four pairs of signals, for example, combining pixels G21 withG22, then combining G23 with G24, then combining G41 with G42, andfinally combining G43 with G44, as shown in FIG. 17E. It is clear thatthere are many additional ways to combine pairs of green signals withincell 220 (FIG. 16). If the sensor does no combining at all, then alleight green signals are reported individually for cell 220. Thus, in thecase of cell 220, the sensor can produce one, two, four or eight greenvalues for cell 220, and produce them in different ways, depending onits mode of operation.

For cells 224, 226, and 228 (FIG. 16), similar color signals areproduced by the sensor depending on its mode of operation. The colorsignals for cells 224, 226, and 228 are red, blue, and green,respectively.

Returning to the case of cell 220, regardless of how many signals aredigitized for this cell, the image processing algorithm of the presentinvention further combines the digitized green values to produce asingle green value for the cell. One way that a single green value isobtained is by averaging all the digitized green values produced forcell 220. In the event that a cell contains color pixels of differingphotoresponses, all the color data within the cell is similarly combinedso that there is a single value for each color photoresponse representedwithin the cell.

It is important to distinguish between the color values pertaining topixels in the original sensor that captured the raw image data, andcolor values pertaining to cells within the original sensor. Both typesof color values are used to produce color images, but the resultingcolor images are of different resolution. An image having pixel valuesassociated with pixels in the original sensor is referred to as ahigh-resolution image, and an image having pixel values associated withcells within the original sensor is referred to as a low-resolutionimage.

Turning now to FIG. 18, the digital signal processor block 36 (FIG. 1)is shown receiving captured raw image data from the data bus 30 (FIG.1). The raw image data is passed to both the Low-resolution PartialColor block 202 and the High-resolution Panchrome block 204. An exampleof a minimal repeating unit for an image sensor has already been shownin FIG. 5 and FIG. 16. In the case of cell 220 (FIG. 16), the capturedraw image data includes the panchromatic data that is produced by theindividual panchromatic pixels as shown in FIG. 17A. Also, for cell 220(FIG. 16), one or more green (color) values are also included, forexample, from the combinations shown in FIGS. 17B-E.

In the Low-resolution Partial Color block 202 (FIG. 18), a partial colorimage is produced from the captured raw image data, a partial colorimage being a color image wherein each pixel has at least one colorvalue and each pixel is also missing at least one color value. Dependingon the sensor's mode of operation, the captured raw data contains somenumber of color values produced by the color pixels within each cell.Within the Low-resolution Partial Color block 202, these color valuesare reduced to a single value for each color represented within thecell. For the cell 220 (FIG. 16), as an example, a single green colorvalue is produced. Likewise, for cells 224, 226 and 228, a single red,blue and green color value is produced, respectively.

The Low-resolution Partial Color block 202 processes each cell in asimilar manner resulting in an array of color values, one for each cell.Because the resulting image array is based on cells rather than pixelsin the original sensor, it is four times smaller in each dimension thanthe original captured raw image data array. Because the resulting arrayis based on cells and because each pixel has some but not all colorvalues, the resulting image is a low-resolution partial color image. Atthis point, the low-resolution partial color image is color balanced.

Looking now at the High-resolution Panchrome block 204, the same rawimage data is used as shown in FIG. 16, although only the panchromaticvalues will be used (FIG. 17A). This time the task is to interpolate acomplete high-resolution panchromatic image by estimating panchromaticvalues at those pixels not having panchromatic values already. In thecase of cell 220 (FIG. 16), panchromatic values must be estimated forthe green pixels in pixel group 222 (FIG. 16). One simple way toestimate the missing panchromatic values is to do vertical averaging.Thus, for example, we can estimate the panchromatic value at pixel 22 asfollows:P22=(P12+P32)/2An adaptive method can also be used. For example, one adaptive method isto compute three gradient values and take their absolute values:SCLAS=ABS(P31−P13)VCLAS=ABS(P32−P12)BCLAS=ABS(P33−P11)using the panchromatic values are shown in FIG. 17A. Likewise, threepredictor values are computed:SPRED=(P31+P13)/2VPRED=(P32+P12)/2BPRED=(P33+P11)/2

Then, set P22 equal to the predictor corresponding to the smallestclassifier value. In the case of a tie, set P22 equal to the average theindicated predictors. The panchromatic interpolation is continuedthroughout the image without regard to cell boundaries. When theprocessing of High-resolution Panchrome block 204 is done, the resultingdigital panchromatic image is the same size as the original captured rawimage, which makes it a high-resolution panchromatic image.

The Low-resolution Panchrome block 206 receives the high-resolutionpanchromatic image array produced by block 204 and generates alow-resolution panchromatic image array which is the same size as thelow-resolution partial color image produced by block 202. Eachlow-resolution panchromatic value is obtained by averaging the estimatedpanchromatic values, within a given cell, for those pixels having colorfilters. In the case of cell 220 (FIG. 16) the high-resolutionpanchromatic values, previously estimated for the green pixels in pixelgroup 222 (FIG. 16), are now averaged together to produce a singlelow-resolution panchromatic value for the cell. Likewise, a singlelow-resolution panchromatic value is computed for cell 224 usinghigh-resolution panchromatic values estimated at the pixels having redfilters. In this manner, each cell ends up with a single low-resolutionpanchromatic value.

The Low-resolution Color Difference block 208 receives thelow-resolution partial color image from block 202 and the low-resolutionpanchrome array from block 206. A low-resolution intermediate colorimage is then formed by color interpolating the low-resolution partialcolor image with guidance from the low-resolution panchrome image. Theexact nature of the color interpolation algorithm, to be discussed indetail later, depends on which pattern of pixel photoresponses was usedto capture the original raw image data.

After the low-resolution intermediate color image is formed it is colorcorrected. Once the low-resolution intermediate color image is colorcorrected, a low-resolution image of color differences is computed bysubtracting the low-resolution panchromatic image from each of thelow-resolution color planes individually. The High-resolution ColorDifference block 210 receives the low-resolution color difference imagefrom block 208 and, using bilinear interpolation, upsamples thelow-resolution color difference image to match the size of the originalraw image data. The result is a high-resolution color difference imagethat is the same size as the high-resolution panchromatic image producedby block 204.

The High-resolution Final Image block 212 receives the high-resolutioncolor difference image from block 210 and the high-resolutionpanchromatic image from block 204. A high-resolution final color imageis then formed by adding the high-resolution panchromatic image to eachof the high-resolution color difference planes. The resultinghigh-resolution final color image can then be further processed. Forexample, it is stored in the DSP Memory block 32 (FIG. 1) and thensharpened and compressed for storage on the Memory Card block 64 (FIG.1).

The sensor filter patterns shown in FIGS. 4A-D have a minimal repeatingunit such that the resulting low-resolution partial color image,produced in block 202, exhibits the repeating Bayer pattern for colorfilters:

$\begin{matrix}G & R \\B & G\end{matrix}$In addition to a single color value, given by the low-resolution partialcolor image, every cell also has a panchromatic value given by thelow-resolution panchromatic image.

Considering the case in which the Bayer pattern is present in thelow-resolution partial color image, the task of color interpolationwithin the Low-resolution Color Differences block 208 (FIG. 18) can nowbe described in greater detail. Color interpolation begins byinterpolating the green values at pixels not already having greenvalues, shown as pixel 234 in FIG. 19A. The four neighboring pixels,shown as pixels 230, 232, 236, and 238, all have green values and theyalso all have panchromatic values. The center pixel 234 has apanchromatic value, but does not have a green value as indicated by thequestion marks.

The first step is to compute two classifier values, the first relatingto the horizontal direction, and the second to the vertical direction:HCLAS=ABS(P4−P2)+ABS(2*P3−P2−P4)VCLAS=ABS(P5−P1)+ABS(2*P3−P1−P5)Then, compute two predictor values, the first relating to the horizontaldirection, and the second to the vertical direction:HPRED=(G4+G2)/2+(2*P3−P2−P4)/2VPRED=(G5+G1)/2+(2*P3−P1−P5)/2Finally, letting THRESH be an empirically determined threshold value, wecan adaptively compute the missing value, G3, according to:

IF MAX( HCLAS, VCLAS ) < THRESH G3 = ( HPRED + VPRED )/2 ELSEIF VCLAS <HCLAS G3 = VPRED ELSE G3 = HPRED ENDThus, if both classifiers are smaller than the threshold value, anaverage of both predictor values is computed for G3. If not, then eitherHPRED or VPRED is used depending on which classifier HCLAS or VCLAS issmaller.

Once all the missing green values have been estimated, the missing redand blue values are interpolated. As shown in FIG. 19B, pixel 242 ismissing a red value but its two horizontal neighbors, pixels 240 and244, have red values R2 and R4 respectively. All three pixels have greenvalues. Under these conditions, an estimate for the red value (R3) forpixel 242 is computed as follows:R3=(R4+R2)/2+(2*G3−G2−G4)/2Missing blue values are computed in a similar way under similarconditions. At this point, the only pixels that still have missing redand blue values are those requiring vertical interpolation. As shown inFIG. 19C, pixel 252 is missing a red value and its two verticalneighbors, pixels 250 and 254, have red values R1 and R5 respectively.Under these conditions, an estimate for the red value (R3) for pixel 252is computed as follows:R3=(R5+R1)/2+(2*G3−G1−G5)/2Missing blue values are computed in a similar way under similarconditions. This completes the interpolation of the low-resolutionpartial color image and the result is a low-resolution intermediatecolor image. As described earlier, the low-resolution color differencescan now be computed by subtracting the low-resolution panchrome valuesfrom each color plane: red, green, and blue in the example justdiscussed.

Turning now to FIG. 20, a partial tiling of a sensor is shown using fourcopies of the minimal repeating unit of FIG. 8A. The four minimalrepeating units 310, 311, 312, and 313 each contain a red, green, andblue pixel. Although the earlier discussion of combining pixels waslimited to like colored pixels within the same minimal repeating unit,as shown in FIG. 16 for example, the present invention can also bepracticed by combining pixels from nearby minimal repeating units. Asshown in FIG. 20, the red pixels R21 and R41 constitute a pair of pixels314 that are combined in a vertical direction. Likewise, the greenpixels G42 and G45 constitute a pair of pixels 315 that are combined ina horizontal direction. When the minimal repeating is relatively small,such as the pattern of FIG. 8A as used in FIG. 20, it is useful tocombine like colored pixels from adjacent minimal repeating units.

It is useful to consider the combining described above with reference toFIG. 20 as occurring between adjacent minimal repeating units, whereadjacency is defined as sharing a boundary of positive length betweentwo minimal repeating units. Given this definition, the minimalrepeating units 311 and 312 in FIG. 20 are adjacent to minimal repeatingunit 310, but minimal repeating units 311 and 312 are not adjacent toeach other. In the combining described above, pixels of like color fromadjacent minimal repeating units are combined, where like color isdefined as having a similar spectral photoresponse. Given thisdefinition, all the red pixels R21, R24, R41, and R44 in FIG. 20 arepixels of like color and all the panchromatic pixels P11 through P16 andP31 through P36 are pixels of like color. In the combining describedabove, similarly positioned pixels from adjacent minimal repeating unitsare combined, where similarly positioned is defined as having the samerelative position within each minimal repeating unit. For example,pixels G42 and G45 in FIG. 20 are similarly positioned and are combinedas pair 315. Note that similarly positioned pixels must be pixels oflike color.

Although combining has generally been described in the context of colorpixels, it is sometimes useful to combine panchromatic pixels. In thecase of low light levels or short exposure times, it is useful tocombine panchromatic pixels to trade off panchromatic resolution toincrease the signal strength of the combined panchromatic pixels. It isimportant to note that the definitions of adjacent minimal repeatingunit, pixels of like color, and similarly positioned pixels apply topanchromatic pixels as well as to color pixels. For example,panchromatic pixels P12 and P15 in FIG. 20 are similarly positioned andare combined as the pair 316. Furthermore, panchromatic pixels P13 andP14 are pixels of like color (but are not similarly positioned) and arecombined as the pair 317. Note that if there are multiple pixels of asingle color within a minimal repeating unit, it is useful sometimes tocombine pixels of like color within a minimal repeating unit. Forexample, panchromatic pixels P35 and P36 are pixels of like color withina minimal repeating unit that are combined as the pair 318.

The combining that has been described to this point includes combiningpixels of like color or similarly positioned pixels from adjacentminimal repeating units. In some cases, it is useful to combinedpanchromatic pixels with color pixels. In FIG. 20, the combined pair 319includes panchromatic pixel P16 and blue pixel B26. Combiningpanchromatic with color pixels is done, for example, when the lightlevel is low or the exposure time is very short in order to increase theoverall signal strength from the combined pixels. Although 319 shows thecombination of a panchromatic pixel and a color pixel from within oneminimal repeating unit, it is also useful to combine panchromatic pixelswith color pixels from nearby minimal repeating units.

It is within the scope of the invention to combine at the same timewithin a given array of minimal repeating units similarly positionedpixels from adjacent minimal repeating units, pixels of like color fromadjacent minimal repeating units, pixels of like color within a minimalrepeating unit, and panchromatic and color pixels. It is also understoodthat a particular arrangement of combining methods is useful at one timedepending on the image capture conditions, and a different arrangementis useful at a different time for different image capture conditions, sothe combining arrangements can be adjusted dynamically at image capturetime. For example, when the overall scene light level is high,panchromatic pixels are not combined, but when the overall scene lightlevel is low, panchromatic pixels are combined with each other or withcolor pixels in order to improve the signal level.

Examples of the various combining methods described above are shown inFIGS. 21 through 27. In FIGS. 21A-21D, two adjacent minimal repeatingunits 330 and 331 that are a variation of the type shown in FIG. 10A areshown. In FIG. 21A, all similarly positioned color pixels from the twominimal repeating units are combined as shown by pixel pairs 332, 333,334, and 335. In FIG. 21B, all similarly positioned panchromatic pixelsare combined as shown by pixels pairs 336, 337, 338, and 339.

FIG. 21C shows combining pixels of like color within a minimal repeatingunit at the same time as combining similarly positioned pixels betweenminimal repeating units. FIG. 21C includes two adjacent minimalrepeating units 330 and 331 that are a variation of the type shown inFIG. 10A. Similarly positioned pixels R21 and R23 are combined as thepair 340, and similarly positioned pixels B42 and B44 are combined asthe pair 341. G22 and G41 are pixels of like color that are located inminimal repeating unit 330, and they are shown combined as the pair 342;similarly, pixels G24 and G43 from minimal repeating unit 331 are showncombined as the pair 343.

FIG. 21D shows combining panchromatic pixels within a minimal repeatingunit at the same time as combining panchromatic pixels between minimalrepeating units. FIG. 21D includes two adjacent minimal repeating units330 and 331 that are a variation of the type shown in FIG. 10A. P11 andP12 are panchromatic pixels that are located in minimal repeating unit330, and they are shown combined as the pair 344; similarly, pixels P13and P14 from minimal repeating unit 331 are shown combined as the pair345. Panchromatic pixels P32 and P33 are in different minimal repeatingunits, and they are shown combined as the pair 346. Uncombinedpanchromatic pixels P31 and P34 are left uncombined or are combined withpixels from adjacent minimal repeating units as shown by the arrows 347and 348.

While the minimal repeating units of FIGS. 21A-21D are arrangedhorizontally, FIG. 22 shows two adjacent minimal repeating units 360 and361 that are arranged vertically, with similarly positioned color pixelscombined as shown by pixel pairs 362, 363, 364, and 365.

In addition to combining pixels in pairs from two adjacent minimalrepeating units, it is useful to combine pixels from three or moreminimal repeating units. FIG. 23, for example, shows three adjacentminimal repeating units 370, 371, and 372 that are a variation of thetype shown in FIG. 10A. Pixels of like color (that are also similarlypositioned) are shown combined in threes as the pixel triplets 373, 374,375, and 376.

FIG. 24 shows five adjacent minimal repeating units 380, 381, 382, 383,and 384 that are a variation of the type shown in FIG. 10A with pixelsof like color shown combined as red triplet 390, green pair 391, greentriplet 392, red pair 393, green triplet 394, blue pair 395, bluetriplet 396, and green pair 397. This particular arrangement producescombined pixel pairs that are evenly spaced. Combination 390 produces acombined red result that is located at the position of R23 (assumingpixels R21, R23, and R25 are equally weighted in the combining process),and combination 391 produces a combined green result that is locatedalso at the position of R23. Similarly, there is a red-green combinedpair at the position of G28, a green-blue combined pair at the positionof G43, and a green-blue combined pair at the position of B48. Pixelpositions R23, G28, G43, and B48 are evenly spaced within the group offive minimal repeating units and, if this group of five minimalrepeating units and combined pixels is repeated to create a larger arrayof minimal repeating units, the resulting combined pixels are evenlyspaced throughout the resulting array. Contrast this with the combiningarrangement shown in FIG. 23: if this group of three minimal repeatingunits and combined pixels is repeated to create a larger array, theresulting combined pixels are concentrated in the center of each groupof three minimal repeating units and are therefore not spaced evenlythroughout the array.

FIG. 25A shows four adjacent minimal repeating units 400, 401, 402, and403 that are a variation of the type shown in FIG. 10A. In FIG. 25A,some pixels of like color are combined within the group of four minimalrepeating units, while other pixels are combined with pixels fromminimal repeating units that are adjacent to this group of four.Specifically, pixels R21, R23, and R25 are combined as the triplet 410,pixels G24, G26, and G28 are combined as the triplet 411, pixels G41,G43, and G45 are combined as the triplet 412, and pixels B44, B46, andB48 are combined as the triplet 413. This leaves several color pixelsthat are left uncombined or are combined with pixels from adjacentminimal repeating units: for example, pixel G22 is combined with pixelsfrom minimal repeating units to the left of the group of four as shownby arrow 414. Similarly, pixel B42 is combined with pixels from adjacentminimal repeating units to the left as shown by the arrow 416, andpixels R27 and G47 are combined with pixels from adjacent minimalrepeating units to the right as shown by arrows 415 and 417,respectively. In FIG. 25A, all similarly positioned color pixels fromthe middle two minimal repeating units, 401 and 402, are combined, withsome of these combined pixels further combined with similarly positionedcolor pixels from a minimal repeating unit to the left, 400, and othercombined pixels further combined with similarly positioned color pixelsfrom a minimal repeating unit to the right, 403.

If a minimal repeating unit of the type exemplified by 400 through 403in FIG. 25A is tiled to create a larger array, the combining arrangementshown in FIG. 25A is extended to cover an array of minimal repeatingunits as shown in FIG. 25B. In FIG. 25B are shown three overlappinggroups of four minimal repeating units: 405L, 405C, and 405R. Group 405Lis composed of minimal repeating units 400L, 401L, 402L, and 403L thatcorrespond for combining purposes to minimal repeating units 400, 401,402, and 403 respectively of FIG. 25A. Similarly, group 405C is composedof minimal repeating units 400C, 401C, 402C, and 403C that alsocorrespond to minimal repeating units 400, 401, 402, and 403 of FIG.25A, and group 405R is composed of minimal repeating units 400R, 401R,402R, and 403R that also correspond to minimal repeating units 400, 401,402, and 403 of FIG. 25A. Note that minimal repeating unit 403L of group405L is also minimal repeating unit 400C of group 405C, and minimalrepeating unit 403C of group 405C is also minimal repeating unit 400R ofgroup 405R. Hence, minimal repeating unit 403L/400C is where groups 405Land 405C overlap, and minimal repeating unit 403C/400R is where groups405C and 405R overlap. The curved lines contained within each group offour minimal repeating units, 405L, 405C, and 405R, connect the pixelsthat are combined as shown by 410, 411, 412, and 413 in FIG. 25A. Thecurved lines that extend outside a group connect to an adjacent andoverlapping group and correspond to 414, 415, 416, and 417 in FIG. 25A.It is clear that the combining arrangement shown in FIGS. 25A and 25Bprovides combined results for the color pixels that are evenly spaced.Furthermore, the combined color pixels represent a Bayer arrangement ofcolors.

Note that the minimal repeating unit is the tiling unit that is used tocreate larger arrays of pixels. The groups of minimal repeating unitsshown in FIGS. 24, 25A, and 25B are for combining purposes only, not fortiling purposes. In some cases the combining is contained entirelywithin the group of minimal repeating units, as shown in FIG. 24, and insome cases the combining extends outside the group as shown in FIG. 25A.In some cases where the combining extends outside a group of minimalrepeating units, representative groups of minimal repeating units areoverlapped for combining purposes as shown in FIG. 25B. Note thatminimal repeating units are grouped in one arrangement for combiningsome pixels, and are group in another arrangement for combining otherpixels. For example, minimal repeating units are grouped in onearrangement to combine color pixels and are grouped in a differentarrangement to combine panchromatic pixels.

Although the examples in the foregoing paragraphs have used minimalrepeating units that are variations of the type shown in FIG. 10A, it isclearly understood that these combining arrangements are used with othertypes of minimal repeating units. For example, replacing the minimalrepeating units in FIGS. 21 through 25 with minimal repeating units thatare variations of the type shown in FIG. 11 produces similar combinedresults and is entirely within the scope of this invention.

The foregoing examples have all shown minimal repeating units that arearranged linearly, either in rows or columns. It is also useful toarrange adjacent minimal repeating units in other ways for the purposeof combining pixels. FIG. 20 provides one such example in which fourminimal repeating units are arranged in two rows and two columns Anotherexample is shown in FIG. 26A, which has three minimal repeating units420, 421, and 422 of the type shown in FIG. 8A. The three minimalrepeating units are arranged so that each minimal repeating unit isadjacent to the others. Pixels of similar color within this group ofthree are combined: for example, pixels R21, R41, and R44 are combined.This arrangement of three mutually adjacent minimal repeating units isrepeated to produce a larger array of pixels. As another example ofmutual adjacency within a group of minimal repeating units, FIG. 26B hasfour minimal repeating units 425, 426, 427, and 428 of the type shown inFIG. 8A, with each of the four minimal repeating units adjacent to atleast two others within the group of four. Pixels of similar color withthis group of four are combined: for example, pixels R21, R41, R44, andR61 are combined.

FIG. 27 shows combining panchromatic pixels with color pixels. Minimalrepeating unit 430 is a variation of the type shown in FIG. 10A. PixelsP11 and R21 form the combined pair 431, pixels P12 and G22 form thecombined pair 432, pixels P31 and G41 form the combined pair 433, andpixels P32 and B42 form the combined pair 434.

Turning now to FIG. 28, a minimal repeating unit having 16 pixels isshown having two copies, minimal repeating units 440 and 441. The topminimal repeating unit 440 is subdivided into two cells 442 and 443.Cell 442 contains a horizontal row of four panchromatic pixels P11, P12,P13, and P14, two blue pixels B21 and B23, and two green pixels G22 andG24. Cell 443 contains a horizontal row of four panchromatic pixels P15,P16, P17, and P18, two red pixels R25 and R27 and two green pixels G26and G28. The bottom minimal repeating unit 441 is subdivided into twocells 444 and 445 that contain similar patterns of pixels as cells 442and 443, respectively. Given a minimal repeating unit that includescells as shown in FIG. 28, there are several ways to combine pixels. InFIG. 28, pixels B21 and B41 from adjacent minimal repeating units 440and 441 provide the combined pixel pair 450. Pixels G42 and G44 from thecell 444 provide the combined pixel pair 451. Pixels P14 and P15 arepanchromatic pixels from cells 442 and 443 respectively that provide thepixel pair 452. Panchromatic pixel P13 is combined with color pixel B23to provide the combined pixel pair 453.

As has been previously shown, the present invention includes combiningan arbitrary number of pixels, both within a single minimal repeatingunit and among multiple adjacent minimal repeating units. As shown inFIG. 28, the green pixels G26, G28, G46, and G48 constitute a four-tupleof combined pixels 455 all of which are combined to produce a singlecolor value. This four-fold combination simultaneously includescombining pixels horizontally and vertically, as well as combining aplurality of pixels from within a single minimal repeating unit, andpixels taken from multiple adjacent minimal repeating units.

Each of FIGS. 29A-C shows a minimal repeating unit 460 of the type shownin FIG. 28. The minimal repeating unit 460 is composed of two cells, 462and 464. In FIG. 29A, pixels B21 and B23 from within cell 462 providethe combined pixel pair 465. Similarly, pixels R25 and R27 from withincell 464 provide the combined pixel pair 466. Pixels G22 and G24 fromwithin cell 462 and combined with pixel G26 from cell 464 to provide thecombined pixel triplet 467. The red and blue combined pixels arecontained entirely within their respective cells, while the greencombined pixels come from two different cells. This particular combiningarrangement has an advantage with respect to placement of the combinedresults: the centroid of the combined blue pixel pair 465 is at theposition of pixel G22, the centroid of the combined green pixel triplet467 is at the position of pixel G24, the centroid of the combined redpixel pair 466 is at the position of pixel G26, while the uncombinedgreen pixel G28 remains at its current position. Hence, the combinedcolor pixels' centroids along with the uncombined green pixel are evenlyspaced, thereby minimizing aliasing in the sampling process.

FIG. 29B shows combining panchromatic pixels from within the cells ofminimal repeating unit 460. Panchromatic pixels P11, P12, and P13provide the combined pixel triplet 468, and panchromatic pixels P15,P16, and P17 provide the combined pixel triplet 469. As with thearrangement of combining color pixels in FIG. 29A, the combiningarrangement of FIG. 29B has an advantage with respect to placement ofthe combined results: the centroid of the combined pixel triplet 468 isat the position of pixel P12, the centroid of the combined pixel triplet469 is at the position of pixel P16, while the uncombined panchromaticpixels P14 and P18 remain at their current positions. Hence, thecombined panchromatic pixels' centroids along with the uncombinedpanchromatic pixels are evenly spaced. Furthermore, these evenly spacedcombined and uncombined panchromatic pixels are arranged verticallyabove the corresponding combined and uncombined color pixels shown inFIG. 29A, potentially simplifying the interpolation process.

FIG. 29C shows additional combinations of panchromatic pixels: pixelsP13 and P14 from cell 462 are combined with pixel P15 from cell 464 toprovide the combined panchromatic triplet 470; pixels P17 and P18 fromcell 464 are combined with the leftmost pixel from a minimal repeatingunit to the right of minimal repeating unit 460 to form the combinedpanchromatic pixel triplet 471; and pixel P11 from cell 462 is combinedwith the rightmost two panchromatic pixels from a minimal repeating unitto the left of minimal repeating unit 460 to form the combinedpanchromatic pixel triplet 472. As with the combined and uncombinedpanchromatic pixels in FIG. 29B, the four combined panchromatic pixeltriplets in FIG. 29C have their centroids evenly spaced. FIG. 29demonstrates that individual pixels are used in multiple combinations aswell as being used only once: Pixel P11 is used twice, once incombination 472 and once in combination 468; pixel P13 is used incombinations 468 and 470; pixel P15 is used in combinations 470 and 469,and pixel P17 is used in combinations 469 and 471.

Not all sensors produce low-resolution partial color images exhibiting arepeating Bayer pattern of color values. For example, the sensor patternshown in FIG. 28 and FIGS. 29A-C determines that two adjacent minimalrepeating units generate two pairs of color values. In each case, a pairof cells from adjacent minimal repeating units produces color values fora low-resolution color representation of the image. The pair of cells442 and 444 provide a blue and a green color value by combining pixelsB21, B23, B41, and B43 for the blue value and by combining pixels G22,G24, G42, and G44 for the green value. Likewise, the pair of cells 443and 445 provide a red and a green color value by combining pixels R25,R27, R45, and R47 for the red value and by combining pixels G26, G28,G46, and G48 for the green value. This pattern of combining pixelsbetween cells from adjacent minimal repeating units is repeated over theentire sensor. The result is a low-resolution representation of theimage wherein each low-resolution pixel has a green color value andeither a red or a blue color value. At this point, the colorinterpolation task within the Low-resolution Color Differences block 208(FIG. 18) estimates missing values of red or missing values of blue foreach pixel. Referring to FIG. 19D, a pixel 264 is shown having a greenvalue (G3) but not having a red value (R3). Four of the neighboringpixels 260, 262, 266, and 268 have green values and red values. Themethod for interpolating the red value for pixel 264 (FIG. 19D) issimilar to the method used to interpolate the green value for pixel 234(FIG. 19A).

The first step is to compute two classifier values, the first relatingto the horizontal direction, and the second to the vertical direction:HCLAS=ABS(G4−G2)+ABS(2*G3−G2−G4)VCLAS=ABS(G5−G1)+ABS(2*G3−G1−G5)Then, compute two predictor values, the first relating to the horizontaldirection, and the second to the vertical direction:HPRED=(R4+R2)/2+(2*G3−G2−G4)/2VPRED=(R5+R1)/2+(2*G3−G1−G5)/2Finally, letting THRESH be an empirically determined threshold value,the missing value G3 is computed adaptively according to:

IF MAX( HCLAS, VCLAS ) < THRESH R3 = ( HPRED + VPRED )/2 ELSEIF VCLAS <HCLAS R3 = VPRED ELSE R3 = HPRED ENDThus, if both classifiers are smaller than the threshold value, anaverage of both predictor values is computed for R3. If not, then eitherHPRED or VPRED is used depending on which classifier HCLAS or VCLAS issmaller.

The missing blue values are interpolated in exactly the same way usingblue values in place of red. Once completed, the low-resolutionintermediate color image has been produced. From there, thelow-resolution color differences are computed as previously described.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications are effected within the spirit and scope ofthe invention.

PARTS LIST

-   -   10 light from subject scene    -   11 imaging stage    -   12 lens    -   13 neutral density filter    -   14 iris    -   16 brightness sensor    -   18 shutter    -   20 image sensor    -   22 analog signal processor    -   24 analog to digital (A/D) converter    -   26 timing generator    -   28 image sensor stage    -   30 digital signal processor (DSP) bus    -   32 digital signal processor (DSP) memory    -   36 digital signal processor (DSP)    -   38 processing stage    -   40 exposure controller    -   50 system controller    -   52 system controller bus    -   54 program memory    -   56 system memory    -   57 host interface    -   60 memory card interface    -   62 memory card socket    -   64 memory card    -   68 user control and status interface    -   70 viewfinder display    -   72 exposure display    -   74 user inputs    -   76 status display    -   80 video encoder    -   82 display controller    -   88 image display    -   100 minimal repeating unit for Bayer pattern    -   102 repeating unit for Bayer pattern that is not minimal    -   110 spectral transmission curve of infrared blocking filter    -   112 unfiltered spectral photoresponse curve of sensor    -   114 red photoresponse curve of sensor    -   116 green photoresponse curve of sensor    -   118 blue photoresponse curve of sensor    -   120 first green cell    -   122 red cell    -   124 blue cell    -   126 second green cell    -   202 low-resolution partial color block    -   204 high-resolution panchromatic block    -   206 low-resolution panchromatic block    -   208 low-resolution color differences block    -   210 high-resolution color differences block    -   212 high-resolution final image block    -   220 first green cell    -   222 green pixels in first green cell    -   224 red cell    -   226 blue cell    -   228 second green cell    -   230 upper pixel values for interpolating missing green value    -   232 left pixel values for interpolating missing green value    -   234 pixel with missing green value    -   236 right pixel values for interpolating missing green value    -   238 lower pixel values for interpolating missing green value    -   240 left pixel values for interpolating missing red value    -   242 pixel with missing red value    -   244 right pixel values for interpolating missing red value    -   250 upper pixel values for interpolating missing red value    -   252 pixel with missing red value    -   254 lower pixel values for interpolating missing red value    -   260 upper pixel values for interpolating missing red value    -   262 left pixel values for interpolating missing red value    -   264 pixel with missing red value    -   266 right pixel values for interpolating missing red value    -   268 lower pixel values for interpolating missing red value    -   310 upper left minimal repeating unit    -   311 upper right minimal repeating unit    -   312 lower left minimal repeating unit    -   313 lower right minimal repeating unit    -   314 example of combining red pixels from vertically adjacent        minimal repeating units    -   315 example of combining green pixels from horizontally adjacent        minimal repeating units    -   316 example of combining similarly positioned panchromatic        pixels from horizontally adjacent minimal repeating units    -   317 example of combining adjacent panchromatic pixels from        horizontally adjacent minimal repeating units    -   318 example of combining adjacent panchromatic pixels from        within a minimal repeating unit    -   319 example of combining a panchromatic pixel with a color pixel    -   330 left minimal repeating unit    -   331 right minimal repeating unit    -   332 combined similarly positioned red pixels    -   333 combined similarly positioned green pixels    -   334 combined similarly positioned green pixels    -   335 combined similarly positioned blue pixels    -   336 combined similarly positioned panchromatic pixels    -   337 combined similarly positioned panchromatic pixels    -   338 combined similarly positioned panchromatic pixels    -   339 combined similarly positioned panchromatic pixels    -   340 combined similarly positioned red pixels    -   341 combined similarly positioned blue pixels    -   342 combined green pixels from within a minimal repeating unit    -   343 combined green pixels from within a minimal repeating unit    -   344 combined panchromatic pixels from within a minimal repeating        unit    -   345 combined panchromatic pixels from within a minimal repeating        unit    -   346 combined panchromatic pixels from between two minimal        repeating units    -   347 panchromatic pixel combined with a pixel from an adjacent        minimal repeating unit    -   348 panchromatic pixel combined with a pixel from an adjacent        minimal repeating unit    -   360 upper minimal repeating unit    -   361 lower minimal repeating unit    -   362 combined similarly positioned red pixels    -   363 combined similarly positioned green pixels    -   364 combined similarly positioned green pixels    -   365 combined similarly positioned blue pixels    -   370 left minimal repeating unit    -   371 center minimal repeating unit    -   372 right minimal repeating unit    -   373 combined similarly positioned red pixels    -   374 combined similarly positioned green pixels    -   375 combined similarly positioned green pixels    -   376 combined similarly positioned blue pixels    -   380 left minimal repeating unit    -   381 center-left minimal repeating unit    -   382 center minimal repeating unit    -   383 center-right minimal repeating unit    -   384 right minimal repeating unit    -   390 combining similarly positioned red pixels    -   391 combined similarly positioned green pixels    -   392 combined similarly positioned green pixels    -   393 combining similarly positioned red pixels    -   394 combined similarly positioned green pixels    -   395 combined similarly positioned blue pixels    -   396 combined similarly positioned blue pixels    -   397 combined similarly positioned green pixels    -   400 left minimal repeating unit    -   401 center-left minimal repeating unit    -   402 center-right minimal repeating unit    -   403 right minimal repeating unit    -   410 combined similarly positioned red pixels    -   411 combined similarly positioned green pixels    -   412 combined similarly positioned green pixels    -   413 combined similarly positioned blue pixels    -   414 green pixel combined with a pixel from an adjacent minimal        repeating unit    -   415 red pixel combined with a pixel from an adjacent minimal        repeating unit    -   416 blue pixel combined with a pixel from an adjacent minimal        repeating unit    -   417 green pixel combined with a pixel from an adjacent minimal        repeating unit    -   400L left minimal repeating unit from left group    -   401L center-left minimal repeating unit from left group    -   402L center-right minimal repeating unit from left group    -   403L right minimal repeating unit from left group    -   400C left minimal repeating unit from center group    -   401C center-left minimal repeating unit from center group    -   402C center-right minimal repeating unit from center group    -   403C right minimal repeating unit from center group    -   400R left minimal repeating unit from right group    -   401R center-left minimal repeating unit from right group    -   402R center-right minimal repeating unit from right group    -   403R right minimal repeating unit from right group    -   405L left group of four minimal repeating units    -   405C center group of four minimal repeating units    -   405R right group of four minimal repeating units    -   420 upper minimal repeating unit    -   421 lower left minimal repeating unit    -   422 lower right minimal repeating unit    -   425 upper minimal repeating unit    -   426 middle-left minimal repeating unit    -   427 middle-right minimal repeating unit    -   428 lower minimal repeating unit    -   430 minimal repeating unit    -   431 combined panchromatic and red pixel    -   432 combined panchromatic and green pixel    -   433 combined panchromatic and green pixel    -   434 combined panchromatic and blue pixel    -   440 upper minimal repeating unit    -   441 lower minimal repeating unit    -   442 left cell in upper minimal repeating unit    -   443 right cell in upper minimal repeating unit    -   444 left cell in lower minimal repeating unit    -   445 right cell in lower minimal repeating unit    -   450 combined similarly positioned blue pixels    -   451 combined green pixels from within a cell    -   452 combined panchromatic pixels from between two cells in a        minimal repeating unit    -   453 combined panchromatic and blue pixel from within a cell    -   455 combined green pixels from within cells and between cells in        different minimal repeating units    -   460 minimal repeating unit    -   462 left cell in minimal repeating unit    -   464 right cell in minimal repeating unit    -   465 combined blue pixels from within a cell    -   466 combined red pixels from within a cell    -   467 combined green pixels from within a cell and between two        cells    -   468 combined panchromatic pixels from within a cell    -   469 combined panchromatic pixels from within a cell    -   470 combined panchromatic pixels from within a cell and between        two cells    -   471 combined pancharomatic pixels from within a cell combined        with one or more pixels from an adjacent minimal repeating units    -   472 panchromatic pixel combined with one or more pixels from an        adjacent minimal repeating unit

1. A system for capturing a color image, comprising: a) a two-dimensional array having first and second groups of pixels wherein pixels from the first group of pixels have narrower spectral photoresponses than pixels from the second group of pixels and wherein the first group of pixels has individual pixels that have spectral photoresponses that correspond to a set of at least two colors; b) the placement of the first and second groups of pixels defining a pattern that has a minimal repeating unit including at least twelve pixels, the minimal repeating unit having a plurality of non-overlapping cells wherein each cell has at least two pixels representing a specific color selected from the first group of pixels and a plurality of pixels selected from the second group of pixels arranged to permit the reproduction of a captured color image under different lighting conditions; and c) means for combining pixels of like color from at least two of the plurality of cells within the minimal repeating unit.
 2. The system of claim 1 wherein the minimal repeating unit is $\begin{matrix} P & P & P & P & P & P & P & P \\ A & B & A & B & C & B & C & B \end{matrix}$ wherein P represents pixels of the second group, and A represents a first color of pixels of the first group, B represents a second color of pixels of the first group, and C represents a third color of pixels of the first group.
 3. The system of claim 2 wherein the combining means includes combining separately the two A pixels, the leftmost three B pixels, and the two C pixels.
 4. The system of claim 2 wherein the combining means includes combining separately in groups of three starting from the left the first, second, and third P pixels; the third, fourth, and fifth P pixels; and the fifth, sixth, and seventh P pixels.
 5. The system of claim 4 wherein the combining means further includes combining separately the leftmost P pixel with at least one pixel from an adjacent minimal repeating unit to the left; and the rightmost two P pixels with at least one pixel from an adjacent minimal repeating unit to the right.
 6. The system of claim 1 wherein pixels are combined by binning the charge from the pixels, by averaging the voltages produced by the pixels, or by first converting the pixel values to digital numbers and then combining the digital numbers, or combinations thereof.
 7. The system of claim 6 wherein the voltages are averaged by first charging capacitors to the voltages produced by the pixels and then connecting the capacitors together to average the voltages, with the capacitors being of equal sizes to perform a simple average or of differing sizes to perform a weighted average.
 8. A system for capturing a color image, comprising: a) a two-dimensional array having first and second groups of pixels wherein pixels from the first group of pixels have narrower spectral photoresponses than pixels from the second group of pixels and wherein the first group of pixels has individual pixels that have spectral photoresponses that correspond to a set of at least two colors; b) the placement of the first and second groups of pixels defining a pattern that has a minimal repeating unit including at least twelve pixels, the minimal repeating unit having a plurality of non-overlapping cells wherein each cell has at least two pixels representing a specific color selected from the first group of pixels and a plurality of pixels selected from the second group of pixels arranged to permit the reproduction of a captured color image under different lighting conditions; and c) means for combining pixels from at least two adjacent minimal repeating units.
 9. The system of claim 8 wherein pixels are combined by binning the charge from the pixels, by averaging the voltages produced by the pixels, or by first converting the pixel values to digital numbers and then combining the digital numbers, or combinations thereof.
 10. The system of claim 9 wherein the voltages are averaged by first charging capacitors to the voltages produced by the pixels and then connecting the capacitors together to average the voltages, with the capacitors being of equal sizes to perform a simple average or of differing sizes to perform a weighted average.
 11. A system for capturing a color image, comprising: a) a two-dimensional array having first and second groups of pixels wherein pixels from the first group of pixels have narrower spectral photoresponses than pixels from the second group of pixels and wherein the first group of pixels has individual pixels that have spectral photoresponses that correspond to a set of at least two colors; b) the placement of the first and second groups of pixels defining a pattern that has a minimal repeating unit including at least twelve pixels, the minimal repeating unit having a plurality of non-overlapping cells wherein each cell has at least two pixels representing a specific color selected from the first group of pixels and a plurality of pixels selected from the second group of pixels arranged to permit the reproduction of a captured color image under different lighting conditions; c) means for combining first group pixels with second group pixels.
 12. The system of claim 11 wherein pixels are combined by binning the charge from the pixels, by averaging the voltages produced by the pixels, or by first converting the pixel values to digital numbers and then combining the digital numbers, or combinations thereof.
 13. The system of claim 12 wherein the voltages are averaged by first charging capacitors to the voltages produced by the pixels and then connecting the capacitors together to average the voltages, with the capacitors being of equal sizes to perform a simple average or of differing sizes to perform a weighted average. 